The present invention relates to a method and apparatus for directional radio communication in which signals between a first station and a second station may be transmitted only in certain directions. In particular, but not exclusively, the present invention is applicable to cellular communication networks using space division multiple access.
With currently implemented cellular communication networks, a base transceiver station (BTS) is provided which transmits signals intended for a given mobile station (MS), which may be a mobile telephone, throughout a cell or cell sector served by that base transceiver station. However, space division multiple access (SDMA) systems have now been proposed. In a space division multiple access system, the base transceiver station will not transmit signals intended for a given mobile station throughout the cell or cell sector but will only transmit the signal in the beam direction from which a signal from the mobile station is received. SDMA systems may also permit the base transceiver station to determine the direction from which signals from the mobile station are received.
SDMA systems may allow a number of advantages over existing systems to be achieved. In particular, as the beam which is transmitted by the BTS may only be transmitted in a particular direction and accordingly may be relatively narrow, the power of the transceiver can be concentrated into that narrow beam. It is believed that this results in a better signal to noise ratio with both the signals transmitted from the base transceiver station and the signals received by the base transceiver station. Additionally, as a result of the directionality of the base transceiver station, an improvement in the signal to interference ratio of the signal received by the base transceiver station can be achieved. Furthermore, in the transmitting direction, the directionality of the BTS allows energy to be concentrated into a narrow beam so that the signal transmitted by the BTS can reach far away located mobile stations with lower power levels than required by a conventional BTS. This may allow mobile stations to operate successfully at greater distances from the base transceiver station which in turn means that the size of each cell or cell sector of the cellular network can be increased. As a consequence of the larger cell size, the number of base stations which are required can also be reduced leading to lower network costs. SDMA systems generally require a number of antenna elements in order to achieve the required plurality of different beam directions in which signals can be transmitted and received. The provision of a plurality of antenna elements increases the sensitivity of the BTS to received signals. This means that larger cell sizes do not adversely affect the reception of signals by the BTS from mobile stations.
SDMA systems may also increase the capacity of the system, that is the number of mobile stations which can be simultaneously supported by the system is increased. This is due to the directional nature of the communication which means that the BTS will pick up less interference from mobile stations in other cells using the same frequency. The BTS will generate less interference to other mobile stations in other cells using the same frequency when communicating with a given MS in the associated cell.
Ultimately, it is believed that SDMA systems will allow the same frequency to be used simultaneously to transmit to two or even more different mobile stations which are arranged at different locations within the same cell. This can lead to a significant increase in the amount of traffic which can be carried by cellular networks.
SDMA systems can be implemented in analogue and digital cellular networks and may be incorporated in the various existing standards such as GSM, DCS 1800, TACS, AMPS and NMT. SDMA systems can also be used in conjunction with other existing multiple access techniques such as time division multiple access (TDMA), code division multiple access (CDMA) and frequency division multiple access (FDMA) techniques.
One problem with SDMA systems is that the direction in which signals should be transmitted to a mobile station needs to be determined. In certain circumstances, a relatively narrow beam will be used to send a signal from a base transceiver station to a mobile station. Therefore, the direction of that mobile station needs to be assessed reasonably accurately. As is known, a signal from a mobile station will generally follow several paths to the BTS. Those plurality of paths are generally referred to as multipaths. A given signal which is transmitted by the mobile station may then be received by the base transceiver station from more than one direction due to these multipath effects.
In general, the decision as to the beam direction which is to be used by the BTS in order to transmit a signal to a mobile station is based on information corresponding to the data burst previously received by the BTS from the given MS. As the decision is based on information received corresponding to only one burst, problems may occur if, for example, the data burst transmitted by the mobile station is superimposed with strong interference.
An additional problem is that the direction in which a signal is to be transmitted by the BTS to the mobile station is determined on the basis of the uplink signals received by the BTS from the mobile station. However, the frequencies of the down link signals transmitted from the mobile station to the BTS are different from the frequencies used for the signals transmitted by the BTS to the mobile station. The difference in the frequencies used in the uplink and downlink signals means that the behaviour of the channel in the uplink direction may be different from the behaviour of the channel in the downlink direction. Thus the optimum direction determined for the uplink signals will not always be the optimum direction for the downlink signals.
A method of transmitting a pilot signal in a code division multiple access cellular radio system is disclosed in WO 96/37969. The method involves receiving at a first station a plurality of signals from a second station, determining a value of a parameter for each received signal and selecting the value of a parameter for a signal to be transmitted in dependence on the value of the parameter of the received signals. This method searches from the best signal continuously and determines the nature of the radio environment by means of a plurality of phasing means. A method of directional radio communication based on similar principles is disclosed in U.S. Pat. No. 5,515,378.
It is therefore an aim of certain embodiments of the present invention to address these difficulties.
According to a first aspect of the present invention, there is provided a method of directional radio communication between a first station and a second station, said method comprising the steps of:
receiving at said first station a plurality of consecutive signals from said second station, said signals each being receivable from at least one of a plurality of different directions;
determining a value of at least one parameter for each of a plurality of sequential signals from the consecutive signals which are received by the first station from the second station; and
selecting a value of at least one parameter for a signal to be transmitted from said first station to said second station, said value of the at least one parameter of the signal to be transmitted by the first station being selected in dependence on said determined value of said at least one parameter of said plurality of sequential signals, wherein said selecting step comprises applying a weighting pattern to said plurality of sequential signals.
By basing a parameter of the signal to be transmitted from the second station to the first station on the parameter of a plurality of signals previously received by the first station, the problems caused by, for example, strong interference in the most recently received signal can be reduced.
Preferably, said step of determining a value of at least one parameter comprises determining the or each direction for each of the plurality of sequential signals and said selecting step comprises selecting at least one direction for the transmission of a signal from the first station to the second station, said at least one direction being selected in dependence on the determined directions for said plurality of sequential signals. By basing the or each direction in which a signal is to be transmitted by the first station to the second station on a plurality of signals received from the second station, the probability that a signal transmitted by the first station will be received by the second station is increased.
Alternatively and/or additionally said step of determining a value of at least one parameter comprises determining the strength of each of said plurality of sequential signals and said selecting step comprises selecting the strength of the signal to be transmitted to said second station, the strength of said signal being selected in dependence on said determined strengths for said plurality of sequential signals. By basing the strength of the signal to be transmitted by the first station to the second station on a plurality of signals received from the second station, the probability that the signal strength will be at the right level is increased. If the signal strength is too low, the second station may not receive the signal, whilst if the signal strength is too high, the risk of interference is unnecessarily increased. It should be appreciated that in those embodiments of the invention, where the signal is transmitted in a plurality of different directions, the strength of the signal in those different directions may differ.
Preferably, the selecting step comprises applying a weighting pattern to said plurality of sequential signals. The term weighting pattern also includes algorithms which provide a weighting function. The weighting pattern may be a uniform weighting pattern so that each of said plurality of sequential signals is given equal weight. The weighting pattern alternatively may be such that the more recently received ones of said plurality of sequential signals are given more weight than the less recently received ones of said plurality of sequential signals. The weighting pattern may be an exponential or linearly increasing weighting pattern. These are just two examples of possible patterns. Any other suitable pattern can be used. Alternatively, the weighting pattern may be defined by an algorithm. It should be appreciated that in some embodiments the weighting pattern is applied to values determined for said parameter.
Preferably, the selecting means selects one of a plurality of weighting patterns in dependence on the radio environment. The weighting patterns may be as outlined previously. For example, in static or slowly changing radio environments, the uniform weighting pattern may be used since it can be expected that the determined the or each direction or strength of the plurality of sequential signals will remain generally the same for those plurality of consecutive signals. Alternatively, if the radio environment is a fast changing radio environment, then the linearly increasing or exponential weighting pattern can be used. With these latter patterns, the previously determined the or each direction or strength of the plurality of received sequential signals will have a negligible influence on the selected beam direction.
Preferably, the method further comprises the step of defining at the first station a plurality of beam directions for transmitting a radiation beam, wherein each of said beam directions is individually selectable.
According to the second aspect of the present invention, there is provided a first station for directional radio communication with a second station, said first station comprising:
receiver means for receiving a plurality of consecutive signals from said second station, said signals each being receivable from at last one of a plurality of different directions;
determining means for determining the value of at least one parameter for each of a plurality of sequential signals from the consecutive signals which are received by the first station from the second station;
transmitter means for transmitting a signal from the first station to the second station; and
control means for controlling said transmitter means, said control means being arranged to select a value of at least one parameter for the signal to be transmitted by said transmitter means, said value of at least one parameter being selected in dependence on the determined values of said at least one parameter for said plurality of sequential signals, wherein said control means is arranged to apply a weighting pattern to said plurality of sequential signals.
Preferably, said determining means is arranged to determine the or each direction for each of the plurality of sequential signals and the control means is arranged to select at least one direction for the transmission of the signal by the transmitter means, said at least one direction being selected in dependence on the determined the or each direction for said plurality of sequential signals. Alternatively or additionally said determining means is arranged to determine the strength of each of said plurality of sequential signals and the control means is arranged to select the strength of the signal to be transmitted by the transmitter means, the strength of the signal being selected in dependence on the determined strength for said plurality of sequential signals.
The control means may be arranged to apply a weighting pattern to said plurality of sequential signals.
Storage means may be provided for storing said determined parameters for each of the plurality of sequential signals.
The receiver means and the transmitter means may comprise an antenna array which is arranged to provide a plurality of signal beams in a plurality of different directions. The antenna array may comprise a phased antenna array or may comprise a plurality of separate antenna elements each of which is arranged to provide a beam in a defined direction. Two separate arrays may be provided, one to receive signals and the other to transmit signals. Alternatively, a single array may be provided both to receive and to transmit signals.
The transmitter means may be arranged to provide a radiation beam in a plurality of beam directions, wherein each of the beam directions is individually selectable. Preferably, the strength of each of the beam directions is individually selectable.
The present invention is particularly applicable to cellular communication networks. In such networks the first station may be a base transceiver station and the second station may be a mobile station respectively. However, it should be appreciated that embodiments of the invention may be applicable to any other type of radio communication network where both the first and second stations may be both stationary or both mobile.